{
  "slug": "unvaccinated-child-measles",
  "question": "What are the odds of an unvaccinated child getting measles?",
  "category": "kids",
  "tags": [
    "child"
  ],
  "no_reliable_estimate": false,
  "perceived": {
    "description": "Measles occupies a peculiar position in public risk perception. Older generations remember it as a routine childhood illness -- unpleasant but survivable. Younger parents, raised in the post-elimination era, often underestimate its contagiousness while simultaneously hearing alarming outbreak headlines. Anti-vaccine communities sometimes frame measles as mild; pro-vaccine messaging emphasizes worst-case complications. The result is a bimodal perception where the actual transmission probability is rarely discussed in concrete terms.\n",
    "rough_estimate": "~5-20% chance an unvaccinated child catches measles",
    "kind": "intuition"
  },
  "native": {
    "display": ">90% infection rate among unvaccinated children by age 15 (pre-vaccine era)",
    "numerator": 90,
    "denominator": 100,
    "unit": "cumulative infection rate by age 15 in a fully susceptible population",
    "population": "unvaccinated children in pre-vaccine-era United States"
  },
  "normalized": {
    "lifetime_us_adult": 0.5,
    "display": "~50% probability an unvaccinated US child contracts measles by age 18 (current era with declining herd immunity)",
    "log_value": -0.3,
    "assumptions": "In the pre-vaccine era (before 1963), >90% of US children were infected with measles by age 15 (CDC Pink Book, Chapter 13). In the current US environment, herd immunity from high (though declining) vaccination rates dramatically reduces -- but does not eliminate -- exposure risk. National MMR coverage has fallen from 95.2% in 2019-2020 to 92.5% in 2024-2025, below the ~95% threshold needed for reliable herd immunity. The 2025 US outbreak saw 2,144 cases with 93% among unvaccinated individuals. With R0 of 12-18, measles is extraordinarily contagious; 90% of susceptible contacts become infected upon exposure. We estimate a current-era cumulative probability of ~50% for an unvaccinated child contracting measles by age 18, reflecting the reality that herd immunity still provides substantial (but eroding) indirect protection, while pockets of low vaccination create significant outbreak risk. This is far below the pre-vaccine >90% but far above what most parents assume.\n",
    "uncertainty": {
      "low": 0.1,
      "high": 0.9
    },
    "scope": "subgroup_lifetime"
  },
  "sources": [
    {
      "url": "https://www.cdc.gov/pinkbook/hcp/table-of-contents/chapter-13-measles.html",
      "title": "Chapter 13: Measles",
      "publisher": "CDC Epidemiology and Prevention of Vaccine-Preventable Diseases (Pink Book)",
      "source_type": "govt_report",
      "statistic": "Before 1963, >90% of persons had measles by age 15; 3-4 million cases/year; ~500 deaths/year in the US",
      "excerpt": "\"Before 1963, approximately 500,000 cases and 500 measles deaths were reported annually, with epidemic cycles every 2-3 years. However, the actual number of cases was estimated at 3-4 million annually. More than 50% of persons had measles by age 6, and more than 90% by age 15.\"\n",
      "source_date": "2024-05-08",
      "source_accessed": "2026-04-18",
      "archive_url": "http://web.archive.org/web/20260412064018/https://www.cdc.gov/pinkbook/hcp/table-of-contents/chapter-13-measles.html",
      "calculation_notes": "CDC Pink Book establishes the pre-vaccine baseline: >90% cumulative infection by age 15 in unvaccinated populations. With 3-4 million annual cases in a US child population of ~50 million, the annual attack rate was roughly 6-8%. Cumulative over 15 years: 1 - (1 - 0.07)^15 ≈ 0.67 to 0.90, consistent with the >90% figure (accounting for epidemic clustering and household transmission).\n"
    },
    {
      "url": "https://www.who.int/news-room/fact-sheets/detail/measles",
      "title": "Measles Fact Sheet",
      "publisher": "World Health Organization",
      "source_type": "reputable_reference",
      "statistic": "R0 of 12-18; 90% of susceptible contacts become infected; caused 136,000 deaths globally in 2022",
      "excerpt": "\"Measles is one of the most contagious diseases in the world. Any person who is not immune (who has not been vaccinated or was vaccinated but did not develop immunity) can get measles.\"\n",
      "source_date": "2024-05-10",
      "source_accessed": "2026-04-18",
      "archive_url": "http://web.archive.org/web/20260412064111/https://www.who.int/news-room/fact-sheets/detail/measles",
      "calculation_notes": "WHO confirms the R0 range of 12-18, making measles the most contagious common infectious disease. The 90% secondary attack rate among susceptible contacts means that in any exposure event, the probability of transmission is near-certain. This underpins the pre-vaccine >90% cumulative infection rate and explains why even small declines in vaccination coverage produce outbreaks.\n"
    },
    {
      "url": "https://pubmed.ncbi.nlm.nih.gov/28757186/",
      "title": "The basic reproduction number (R0) of measles: a systematic review",
      "publisher": "The Lancet Infectious Diseases (Guerra et al.)",
      "source_type": "peer_reviewed",
      "statistic": "Median R0 of 12.5 (range 3.7-203.3) across studies; most estimates cluster between 12-18",
      "excerpt": "\"We identified 58 studies... The median R0 value across studies was 12.5.\"\n",
      "source_date": "2017-12-01",
      "source_accessed": "2026-04-18",
      "archive_url": "http://web.archive.org/web/20260317235151/https://pubmed.ncbi.nlm.nih.gov/28757186/",
      "calculation_notes": "Guerra et al. systematic review of R0 estimates confirms measles as exceptionally transmissible. The R0 range means that in a fully susceptible population, each case generates 12-18 secondary cases on average. This makes herd immunity thresholds very high (1 - 1/R0 = 92-94%), explaining why vaccination coverage drops from 95% to 92.5% can trigger outbreaks.\n"
    },
    {
      "url": "https://www.cdc.gov/measles/data-research/index.html",
      "title": "Measles Cases and Outbreaks",
      "publisher": "CDC",
      "source_type": "govt_report",
      "statistic": "2,144 confirmed measles cases in 2025; 93% among unvaccinated or unknown vaccination status",
      "excerpt": "\"As of May 30, 2025, 2,144 confirmed measles cases and 3 deaths have been reported in the United States.\"\n",
      "source_date": "2025-05-30",
      "source_accessed": "2026-04-18",
      "archive_url": "https://web.archive.org/web/20260421192757/https://www.cdc.gov/measles/data-research/index.html",
      "calculation_notes": "The 2025 outbreak data demonstrates the real-world consequence of declining vaccination. With 2,144 cases and 93% among unvaccinated individuals, this represents the highest case count since measles was declared eliminated in 2000. Three deaths confirm the ongoing lethality of the disease even in a modern healthcare setting.\n"
    }
  ],
  "comparison_anchors": [
    {
      "label": "Unvaccinated child getting chickenpox (pre-vaccine era, lifetime)",
      "lifetime_us_adult": 0.95
    },
    {
      "label": "Unvaccinated child getting whooping cough (current US, by age 18)",
      "lifetime_us_adult": 0.05
    },
    {
      "label": "Appendicitis (lifetime, US adult)",
      "lifetime_us_adult": 0.07
    }
  ],
  "personal_factor_multipliers": [
    {
      "factor": "lives in a community with <90% MMR coverage",
      "multiplier": 3,
      "notes": "Outbreaks cluster in undervaccinated communities; the 2025 US outbreak was concentrated in jurisdictions with below-average MMR uptake"
    },
    {
      "factor": "lives in a community with >95% MMR coverage",
      "multiplier": 0.1,
      "notes": "Robust herd immunity dramatically reduces exposure probability even for unvaccinated individuals"
    },
    {
      "factor": "attends daycare or school with vaccine exemption clusters",
      "multiplier": 5,
      "notes": "Congregate settings with susceptible individuals are the primary transmission venues; school-based outbreaks account for a large share of pediatric cases"
    },
    {
      "factor": "international travel to endemic regions",
      "multiplier": 4,
      "notes": "Most US measles introductions are travel-related; unvaccinated travelers to Africa, South/Southeast Asia face high exposure risk"
    }
  ],
  "short_label": "Unvaxxed child & measles",
  "outcome_severity": "serious_harm",
  "exposure_pattern": "acute",
  "outcome_type": "recoverable_injury",
  "valence": "negative",
  "caveats": "The normalized 50% estimate is necessarily imprecise because it depends heavily on local vaccination coverage, which varies enormously by county and school district. An unvaccinated child in a 98%-vaccinated suburban school faces far lower risk than one in a 75%-vaccinated community. The pre-vaccine >90% figure represents the biological ceiling in a fully susceptible population. The current estimate will continue to shift as vaccination rates change. Measles case fatality in the US is roughly 1-3 per 1,000 cases, but rises significantly in malnourished populations and infants.\n",
  "quality_score": {
    "d1": 5,
    "d2": 5,
    "d3": 3,
    "d4": 3,
    "d5": 5,
    "d6": 5,
    "d7": 4,
    "d8": 4,
    "avg": 4.25,
    "scored_by": "claude-code-8d",
    "scored_at": "2026-05-25",
    "methodology_version": "1.2"
  },
  "reviewer": "quality-review-agent",
  "last_reviewed": "2026-04-19",
  "reviewed": true,
  "generated_at": "2026-04-18",
  "image": {
    "alt": "A single measles virus particle rendered in flat vector style against a plain background, muted red and grey tones."
  },
  "attribution": "Likelier — https://likelier.app",
  "license": "https://creativecommons.org/licenses/by-sa/4.0/",
  "support": "https://buymeacoffee.com/kgluszczyk?via=likelier&utm_content=api-fear-single",
  "canonical_url": "https://likelier.app/unvaccinated-child-measles"
}